I assume you mean in FFox... That is done and in the case of TBird, it
works as you describe. In KMail, it appears that KMail steals the FFox
application to it's own desktop and then FFox opens the new URL in a new
tab, as it should. The problem isn't that a new window is opening, it is
that the existing window(s) and tabs are moved from one desktop to
another.

I believe the problem is in KMail or as some have suggested in KDE
itself.